Deadlock avoidance in a bus fabric

ABSTRACT

Circuits, apparatus, and methods for avoiding deadlock conditions in a bus fabric. One exemplary embodiment provides an address decoder for determining whether a received posted request is a peer-to-peer request. If it is, the posted request is sent as a non-posted request. A limit on the number of pending non-posted requests is maintained and not exceed, such that deadlock is avoided. Another exemplary embodiment provides an arbiter that tracks a number of pending posted requests. When the number pending posted requests reaches a predetermined or programmable level, a Block Peer-to-Peer signal is sent to the arbiter&#39;s clients, again avoiding deadlock.

BACKGROUND

The present invention relates generally to deadlock avoidance in a busfabric, and more particularly to deadlock avoidance at an interfacebetween integrated circuits.

Few applications stress the resources of a computer systems to theextent that video does. Video capture, encoding, and the like involvehuge transfers of data between various circuits in a computer system,for example, between video-capture cards, central processing units,graphics processors, systems memories, and other circuits.

Typically, this data is moved over various buses, such as PCI buses,HyperTransport™ buses, and the like, both on and between the integratedcircuits that form the computer system. Often, first-in-first-outmemories (FIFOs) are used to isolate these circuits from one another,and to reduce the timing constraints of data transfers between them.

But these FIFOs consume expensive integrated circuit die area and power.Accordingly, it is desirable to limit the depth of the FIFOs.Unfortunately, this means that these FIFOs may become filled and notable to accept further inputs, thus limiting system performance.

It is particularly problematic if these filled FIFOs are in a data paththat forms a loop. In that case, there may be a processor, such as agraphics processor, or other circuit in the loop that becomesdeadlocked, that is, unable to either receive or transmit data.

This can happen under the following conditions, for example. A firstFIFO that receives data from a circuit cannot receive data because it isfull. The first FIFO cannot send data to a second FIFO because thesecond FIFO is also full. The second FIFO similarly cannot send databecause it wants to send the data to the circuit, which cannot accept itsince it is waiting to send data to the first FIFO. This unfortunate setof circumstances can result in a stable, deadlocked condition.

Thus, what is needed are circuits, methods, and apparatus for avoidingthese deadlocked conditions. While it may alleviate some deadlockedconditions to increase the size of the FIFOs, again there is anassociated cost in terms of die area and power, and the possibilityremains that an even deeper FIFO may fill. Thus, it is desirable thatthese circuits, methods, and apparatus not rely solely on making theseFIFOs deeper and be of limited complexity.

SUMMARY

Accordingly, embodiments of the present invention provide circuits,apparatus, and methods for avoiding deadlock conditions. One exemplaryembodiment provides an address decoder for determining whether areceived posted write request is a peer-to-peer request. If it is, therequest is converted to a non-posted write request. A limit on thenumber of pending non-posted requests is maintained and not exceeded,such that deadlock is avoided. The number of pending non-posted requestsis tracked by subtracting the number of responses received from thenumber of non-posted requests sent.

Another exemplary embodiment does not convert received posted requeststo non-posted requests, but rather provides an arbiter that that tracksthe number of pending posted requests. When the number of pending postedrequests (for example, the number of pending requests in a FIFO orqueue) reaches a predetermined or programmable level, that is alow-water mark, a Block Peer-to-Peer signal is sent to an arbiter'sclients. This keeps the FIFOs in a data loop from filling, thus avoidingdeadlock. When a response or signal indicating that the number ofpending posted requests is below this level is received by the arbiter,the Block Peer-to-Peer signal is removed, and peer-to-peer requests mayagain be granted. Alternately, the number of pending peer-to-peerrequests may be tracked, and when a predetermined or programmable levelis reached, a Block Peer-to-Peer signal is asserted. Circuits, methods,and apparatus consistent with the present invention may incorporate oneor both of these or the other embodiments described herein.

A further exemplary embodiment of the present invention provides amethod of transferring data. This method includes receiving a transferrequest, determining if the transfer request is a write to a memorylocation, if the transfer request is a write to a memory location, thensending the transfer request as a posted request, otherwise determininga number of available transfer request entries in a posted-requestfirst-in-first-out memory, and if the number of transfer request entriesavailable is greater than a first number, then sending the transferrequest as a posted request, otherwise waiting to send the transferrequest as a posted request.

A further exemplary embodiment of the present invention provides anothermethod of transferring data. This method includes maintaining a firstnumber of tokens, receiving a plurality of posted requests, if aremaining number of the first number of tokens is less than a firstnumber, forwarding one of the plurality of posted requests as anon-posted request, else not forwarding the one of the plurality ofposted requests as a non-posted request.

Yet another exemplary embodiment of the present invention provides Anintegrated circuit. This integrated circuit includes an arbiterconfigured to track a number of available entries in a posted requestFIFO, a plurality of clients coupled to the arbiter, and aHyperTransport bus coupled to the arbiter, wherein the arbiter receivespeer-to-peer requests from the plurality of clients and provides postedrequests to the posted request FIFO, and when the number of availableentries in the posted request FIFO is equal to a first number, thenpreventing the plurality of clients from sending peer-to-peer requests.

A better understanding of the nature and advantages of the presentinvention may be gained with reference to the following detaileddescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a computing system that benefits byincorporation of embodiments of the present invention;

FIG. 2 is a block diagram of an improved computing system that isbenefited by the incorporation of embodiments of the present invention;

FIG. 3 is a simplified block diagram of the improved computingprocessing system of FIG. 2;

FIG. 4 is a further simplified block diagram of the improved computingsystem of FIG. 2 illustrating the write path from a video-capture cardto a system memory;

FIG. 5 is a simplified block diagram of the improved computing system ofFIG. 2 that incorporates an embodiment of the present invention;

FIG. 6 is a flowchart further describing a specific embodiment of thepresent invention;

FIG. 7 is a simplified block diagram of the improved computing system ofFIG. 2 that incorporates an embodiment of the present invention; and

FIG. 8 is a flowchart further describing a specific embodiment of thepresent invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a block diagram of a computing system 100 that benefits byincorporation of embodiments of the present invention. This computingsystem 100 includes a Northbridge 110, graphics accelerator 120,Southbridge 130, frame buffer 140, central processing unit (CPU) 150,audio card 160, Ethernet card 162, modem 164, USB card 166, graphicscard 168, PCI slots 170, and memories 105. This figure, as with all theincluded figures, is shown for illustrative purposes only, and does notlimit either the possible embodiments of the present invention or theclaims.

The Northbridge 110 passes information from the CPU 150 to and from thememories 105, graphics accelerator 120, and Southbridge 130. Southbridge130 interfaces to external communication systems through connectionssuch as the universal serial bus (USB) card 166 and Ethernet card 162.The graphics accelerator 120 receives graphics information over theaccelerated graphics port (AGP) bus 125 through the Northbridge 110 fromCPU 150 and directly from memory or frame buffer 140. The graphicsaccelerator 120 interfaces with the frame buffer 140. Frame buffer 140may include a display buffer that stores pixels to be displayed.

In this architecture, CPU 150 performs the bulk of the processing tasksrequired by this computing system. In particular, the graphicsaccelerator 120 relies on the CPU 150 to set up calculations and computegeometry values. Also, the audio or sound card 160 relies on the CPU 150to process audio data, positional computations, and various effects,such as chorus, reverb, obstruction, occlusion, and the like, allsimultaneously. Moreover, the CPU 150 remains responsible for otherinstructions related to applications that may be running, as well as forthe control of the various peripheral devices connected to theSouthbridge 130.

FIG. 2 is a block diagram of an improved computing system that isbenefited by the incorporation of embodiments of the present invention.This block diagram includes a combined processor and Northbridge 210,media control processor 240, and system memory 270. Also included inthis block diagram for exemplary purposes is a video capture card 280.

The combined processor and Northbridge 210 includes a central processingunit 212, FIFO 216, multiplexer 222, output buffers 224 including onefor posted requests 226, non-posted requests 228, and responses 230,input FIFO 232 including an input FIFO for posted requests 234,non-posted requests 236, and responses 238, address decoder 220,peer-to-peer FIFO 218, and memory controller 214.

The media control processor includes input FIFO 242 for posted requests244, non-posted requests 246, and responses 248, an integrated graphicsprocessor 252, arbiter 250, and PCI-to-PCI bridge 260. The combined CPUand Northbridge 210 communicates with the media control processor 240over HyperTransport buses 290 and 295. The system memory 270 couples tothe memory controller 214 over memory interface bus 272, while the videocapture card 280 is connected to the PCI-to-PCI bridge 260 over the PCIbus 282.

In a specific embodiment of the present invention, the combined CPU andNorthbridge 210 is formed on a first integrated circuit, while the mediacontrol processor 240 is formed on a second integrated circuit. Inanother embodiment, the graphics processor 252 is not integrated on themedia control processor, but is rather a separate integrated circuitcommunicating over an advanced graphics processor (AGP) bus with themedia control processor 240. In other embodiments, these variousfunctions may be divided in other ways and integrated on differentnumbers of integrated circuits communicating over various buses.

Data and requests move between these integrated circuits and integratedcircuit blocks over buses. In the case of a write request, a circuitrequests that it be allowed to place data on a bus, and that request isgranted. The data may either be sent as a posted request, in which noresponse is required, or as a non-posted request, in which case aresponse is required. The response is sent back to the sending circuitafter the write has been completed at its destination circuit.

These different transactions, posted requests, non-posted requests, andresponses, are stored in separate FIFOs as shown. These separate FIFOsmay be the same size, or they may be different sizes. Further, thevarious FIFOs may have different sizes. In one specific embodiment, thenon-posted request FIFO 236 has 6 entries, the peer-to-peer FIFO 218 hastwo entries, and the non-posted request FIFO 228 has 16 entries. Invarious embodiments, the peer-to-peer FIFO 218 may be one FIFO forstoring posted and non-posted requests and responses, or it may beseparate FIFOs for storing the different types of transactions. Moreinformation about these various types of requests and peer-to-peertransactions can be found in the HyperTransport specification, which iscurrently on release 1.05 published by the HyperTransport Consortium,which is incorporated by reference.

In this new architecture, the graphics processor has become separatedfrom the system memory. This separation leads to data paths that canform a loop, and thus become deadlocked. Specifically, data transfersfrom the CPU 212 and video capture card 280 may fill the various FIFOs.

In the configuration shown in FIG. 2, the CPU 212 writes to a framebuffer in the system memory 270 utilizing the following path. The CPU212 provides requests (data), on line 213 to the FIFO 216. The FIFO 216provides data to the multiplexer 222, which in turn provides the data tothe output buffers 224. The buffers 224 provide data over HyperTransportbus 290 to FIFO 242, which in turn provide data to the graphics buses252. The graphics processor 252 provides the requests on line 254 to thearbiter 250. The arbiter 250 provides the requests back over theHyperTransport bus 295 to the FIFO 232. The FIFO 232 provides therequest to the address decoder 220, which in turn provides them to thememory controller 214. The memory controller 214 writes to the systemmemory 270 over memory interface buses 272.

Also in this configuration, the video capture card 218 writes data to aframe buffer in the systems memory 270 utilizing the following path. Thevideo capture card 280 provides data on PCI bus 282 to the PCI-to-PCIbridge 260. The PCI-to-PCI bridge 260 provides data to the arbiter 250,which in turn provides the requests over HyperTransport bus 295 to theFIFO 232. The FIFO 232 provides the requests to the address decoder 220,which in turn provides it to the peer-to-peer FIFO 218. The peer-to-peerFIFO 218 provides the data to multiplexer 222, which in turn provides itto the output buffers 224. The output buffers 224 provide the data tothe FIFO 242, which in turn provides it to the graphics processor 252.

The graphics processor 252 then writes to the frame buffer in thesystems memory 270 utilizing the following path. The graphics processor252 provides modified requests on line 254 to the arbiter 250. Thearbiter 250 provides the data over HyperTransport bus 295 to the FIFO232. FIFO 232 provides the data to the address decoder 220. This time,the address decoder sees a new address provided by the graphicsprocessor 252, and in turn provides the request to the memory controller214. The memory controller 214 then writes the data to the systemsmemory 270 over memory interface buses 272.

As can be seen, this convoluted path crosses the HyperTransportinterface buses 290 and 295 a total of three times. Particularly insituations where the CPU 212 and video capture card 280 are writing to aframe buffer in the systems memory 270, the FIFOs 242, 232, and 218 maybecome full, that is, unable to accept further inputs. In this case, thesituation may arise where the graphics processor 252 tries to write datato the frame buffer in the systems memory 270, but cannot because thearbiter 250 can not grant the graphics processor 252 access to theHyperTransport bus 295. Similarly, the receive FIFO 232 cannot outputdata because the peer-to-peer FIFO 218 is full. Further, thepeer-to-peer FIFO 218 cannot output data because the media controlprocessor input FIFO 242 is similarly full. In this situation, the busfabric is deadlocked and an undesirable steady-state is reached.

FIG. 3 is a simplified block diagram of the improved computingprocessing system of FIG. 2. Included are a combined CPU and Northbridge310, media control processor 340, system memory 370, and video capturecard 380. The combined CPU and Northbridge 310 includes a transmitter312 and receiver 314, while the media control processor includes areceiver 342, transmitter 344, graphics processor 346, and PCI-to-PCIbridge 348. A systems memory 370 communicates with the combined CPU andNorthbridge over a memory interface bus 372. In this particular example,a video capture card 380 is included, which communicates with the mediacontrol processor over a PCI bus 382.

FIG. 4 is a further simplified block diagram of the improved computingsystem of FIG. 2 illustrating the write path from the video-capture card480 to the system memory 470. This block diagram includes a combined CPUand Northbridge 410, media control processor 440, system memory 470, andvideo capture card 480. The combined CPU and Northbridge circuitincludes a transmitter 412 and receiver 414. The media control processorincludes a receiver 442, transmitter 444, graphics processor 446, andPCI-to-PCI bridge 448.

The video capture card 480 provides requests to the PCI-to-PCI bridge448, which in turn provides them to the transmitter 444. The transmitter444 sends requests to the receiver 414, which in turn provides them tothe transmitter 412. The transmitter 412 sends these requests to thereceiver 442, which passes them along to the graphics processor 446. Thegraphics processor 446 writes the data to the systems memory by sendingit as a request to the transmitter 444, which in turn provides it to thereceiver 414. The receiver 414 then writes the data to the systemsmemory 470.

As can be seen, the requests cross from the transmitter 444 to thereceiver 414 twice during this process. This is where the potential fora deadlock arises. Specifically, in the deadlocked condition, thegraphics processor cannot send a request to the transmitter 444, becausethe transmitter cannot send to the receiver 414, since its associatedFIFO is full. The graphics processor cannot accept a new request becauseit is waiting to granted its own request. Accordingly, it cannot drainthe FIFO in the receiver 442. Again, a deadlocked condition arises,creating an undesirable steady-state.

FIG. 5 is a simplified block diagram of the improved computing system ofFIG. 2 that incorporates an embodiment of the present invention. Thisblock diagram includes a combined CPU and Northbridge 510, media controlprocessor 540, systems memory 570, and video card 580. The combined CPUand Northbridge 510 includes a transmitter 512 and a receiver 514. Themedia control processor 540 includes a receiver 542, transmitter 544,graphics processor 546, and PCI-to-PCI bridge 548. The PCI-to-PCI bridge548 further includes an address decoder 562.

A posted request is provided by the video capture card 580 to thePCI-to-PCI bridge 548. The address decoder 562 in the PCI-to-PCI bridge548 determines that this posted request is a peer-to-peer request andconverts it to a non-posted request and passes it to the transmitter544. The transmitter 544 sends this request as a non-posted request thatis sent to the receiver 514. The receiver 514 then sends the request tothe transmitter 512, which passes it to the receiver 542. The receiver542 in turn provides the request to the graphics processor 546.

The graphics processor 546 then reflects the request back upstream tothe transmitter 544 as a posted request having an address in the framebuffer in the system memory 570. The graphics processor also issues a“target done” completion response. The combined CPU and Northbridge 510receive the posted request and response from the transmitter 544. Theposted request is sent to the system memory 570, and the response issent back to the media control processor 540, where it is received bythe PCI-to-PCI bridge 548.

In this embodiment, the number of pending non-posted requests is limitedto some number “N”, such as 1, and when this number is reached, nofurther non-posted requests are provided to the transmitter 544.Specifically, as a non-posted request is sent, a count is incremented inthe address decoder portion 562 of the PCI-to-PCI bridge 548. Asresponses are received by the PCI-to-PCI bridge 548, this count isdecremented. When the count is reached, further non-posted requests areheld by the address decoder 562. This avoids the deadlocked conditiondescribed above.

FIG. 6 is a flowchart further describing this specific embodiment of thepresent invention. In act 610, a posted request is received from a videocapture card. In act 620, the address associated with the request isdecoded and a determination of whether the request is peer-to-peer or tobe written to the system memory is made. If it is not a peer-to-peerrequest, that is, it is data to be written to the system memory, it issent as a posted request in act 680. If it is a peer-to-peer request,the request is converted to a non-posted request in act 630. In act 640,it is determined whether the number of pending non-posted requests isequal to a predetermined or programmable number of allowable pendingnon-posted requests, such as 1 or another number, in act 650. If thecount has not reached this number “N”, the request may be sent as anon-posted request in act 660, and the count is incremented by one inact 670. If the count has reached “N” however, the requests is stalledor not granted in order to avoid a deadlocked condition in act 650. Asnon-posted requests are completed, responses are received and the countis decremented.

Returning to FIG. 2, we can see how this embodiment is implemented ingreater detail. The video capture card 280 provides posted requests onPCI bus 282 to the PCI-to-PCI bridge 260. An address decoder in thePCI-to-PCI bridge 260 determines whether the request is a write to thesystem memory 270. If it is, that request is passed to the arbiter 250which places it in the posted request FIFO 234, which forwards it to thememory controller 214, which writes it to the system memory 270.

If the posted request is a peer-to-peer request, that is, it is not tobe written directly to the system memory 270 but is destined for a peercircuit, for example the graphics processor 252, then the posted requestis converted to a non-posted request by an address decoder (or othercircuit) in the PCI-to-PCI bridge 260. This non-posted request is routedfrom the arbiter 250 to the non-posted request FIFO 236, to thepeer-to-peer FIFO 218. The non-posted request then reaches the graphicsprocessor 252 via bus 290. The graphics processor converts the requestback to a posted request and also issues a response. The posted requestis passed to the memory controller 214 which writes the data to thesystem memory 270, while the response is received by the PCI-to-PCIbridge 260.

The decoder in the PCI-to-PCI bridge 260 also keeps track of the numberof pending non-posted requests, and does not send non-posted requests tothe non-posted request FIFO 236 once it has determined that apredetermined or programmable number of pending non-posted requests hasbeen reached.

FIG. 7 is a simplified block diagram of the improved computing system ofFIG. 2 that incorporates an embodiment of the present invention. Thisblock diagram includes a combined CPU and Northbridge 710, media controlprocessor 740, systems memory 770, and video-capture card 780. Thecombined CPU and Northbridge 710 includes a transmitter 712 and receiver714. The media control processor 740 includes a receiver 742,transmitter 744, graphics processor 746, and PCI-to-PCI bus 748. Thetransmitter 744 further includes an arbiter 745.

Posted requests provided by the video capture card 780 are provided tothe PCI-to-PCI bridge 748, which passes them to the arbiter 745. Thearbiter tracks posted requests (or alternately, peer-to-peer requests)that are pending at the receiver 714. When a certain number of postedrequests remain pending, the arbiter 745 sends out a Block Peer-to-Peersignal to its clients such as the graphics processor 746 and PCI-to-PCIbridge 748. In this case, no further peer-to-peer requests are sent tothe arbiter 745 until a response indicating that there is room in thereceiver 714 posted request FIFO is received by the arbiter 745.

If the Block Peer-to-Peer signal is not asserted, the posted request isprovided to the transmitter 744, which sends it to the receiver 714. Thereceiver 714 routes it to the receiver 742 via the transmitter 712. Thereceiver 742 passes the posted request to the graphics processor 746.The graphics processor 746 in turn passes it to the transmitter 744 tothe receiver 714, which provides it to the system memory 770.

FIG. 8 is a flowchart further describing this specific embodiment of thepresent invention. In act 810, an arbiter receives a posted request, forexample from a video capture card. In act 820, the arbiter determineswhether the posted request is a peer-to-peer request. If it is not, thenin act 830, the data is sent as a posted write request. If it is, thenin act 840, it is determined whether the FIFO is below its low-watermark, or alternatively, whether a block peer-to-peer signal or state hasbeen asserted. If this is true, then in act 850, the arbiter waits foran entry to become available in the posted write FIFO. At some point,the posted write FIFO provides an output, thus freeing up an entry. Atthis time, the arbiter releases the Block Peer-to-Peer signal and thedata is sent to the posted write FIFO in act 830.

Returning to FIG. 2, we can see how this embodiment is implemented ingreater detail. The video capture card 280 provides posted requests onPCI bus 282 to the PCI-to-PCI bridge 260. The PCI-to-PCI bridge 260passes these requests to the arbiter 250. The arbiter keeps track of anumber of pending posted requests in the posted request FIFO 236 (oralternately, the number of pending peer-to-peer requests, or the numberof posted requests in FIFO 218). When the number of pending postedrequests in the posted request FIFO 236 reaches a predetermined orprogrammable level the arbiter 250 broadcasts a Block Peer-to-Peersignal to the graphics processor 252, PCI-to-PCI bridge 260, and otherclient circuits. This keeps those circuits from sending furtherpeer-to-peer requests, thus avoiding a deadlocked condition.

When the number of pending posted requests is below this low-water mark,the posted request is sent to the posted request FIFO 234. The postedrequest is then routed through the peer-to-peer FIFO 218, multiplexer222, FIFOs 226 and 244, to the graphics processor 252. The graphicsprocessor then converts the address to a system memory address 270, andforwards the posted request to the arbiter 250. The arbiter 250 passesthe posted request to the posted request FIFO 234, to the memorycontroller 214, which writes data to the system memory 270.

In one embodiment, at power up, the arbiter 250 receives a number oftokens, for example six tokens. As the arbiter provides a peer-to-peerposted request to the posted request FIFO 234, it sends along one ofthese tokens. As the posted request FIFO outputs a peer-to-peer postedrequest, the arbiter receives a token. If the count of tokens drops to alow-water mark level, for example one, the arbiter 250 asserts the BlockPeer-to-Peer signal. When tokens are received, the Block Peer-to-Peersignal is removed.

The above description of exemplary embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdescribed, and many modifications and variations are possible in lightof the teaching above. The embodiments were chosen and described inorder to best explain the principles of the invention and its practicalapplications to thereby enable others skilled in the art to best utilizethe invention in various embodiments and with various modifications asare suited to the particular use contemplated.

1. A method of transferring data comprising: receiving a transferrequest; determining if the transfer request is a write to a memorylocation; if the transfer request is a write to a memory location, thensending the transfer request as a posted request; else determining anumber of available transfer request entries in a posted-writefirst-in-first-out memory; and if the number of transfer request entriesavailable is greater than a first number; then sending the transferrequest as a posted request; else waiting to send the transfer requestas a posted request.
 2. The method of claim 1 wherein the transferrequest is made by a video capture card.
 3. The method of claim 1wherein the transfer request is made by a graphics processor.
 4. Themethod of claim 1 wherein the transfer request is sent over aHyperTransport bus.
 5. The method of claim 1 wherein the number ofpending posted requests is determined by an arbiter.
 6. The method ofclaim 1 wherein the first number is programmable.
 7. The method of claim1 wherein the first number has a value of one.
 8. A method oftransferring data comprising: maintaining a first number of tokens;receiving a plurality of posted requests; if a remaining number of thefirst number of tokens is less than a first number, forwarding one ofthe plurality of posted requests as a non-posted request; else notforwarding the one of the plurality of posted requests as a non-postedrequest.
 9. The method of claim 8 wherein the first number of tokens isone.
 10. The method of claim 8 wherein the first number of tokens ismaintained by an address decoder.
 11. The method of claim 8 furthercomprising: receiving a response from a pending non-posted request; andincrementing the number of available tokens by one.
 12. The method ofclaim 11 further comprising when the number of tokens is incrementedabove the first number, then forwarding the one of the plurality ofposted requests as a non-posted request.
 13. An integrated circuitcomprising: an arbiter configured to track a number of available entriesin a posted request FIFO; a plurality of clients coupled to the arbiter;and a HyperTransport bus coupled to the arbiter; wherein the arbiterreceives peer-to-peer requests from the plurality of clients andprovides posted requests to the posted request FIFO, and when the numberof available entries in the posted request FIFO is equal to a firstnumber, then preventing the plurality of clients from sendingpeer-to-peer requests.
 14. The integrated circuit of claim 13 whereinthe plurality of clients includes a graphics processor.
 15. Theintegrated circuit of claim 14 wherein the plurality of clients furtherincludes a PCI-to-PCI bridge.
 16. The integrated circuit of claim 13wherein the number of pending peer-to-peer requests is incremented byone for each granted posted request.
 17. The integrated circuit of claim16 wherein the number of pending peer-to-peer requests is decremented byone for each peer-to-peer request provided by a receive FIFO.
 18. Theintegrated circuit of claim 13 further comprising when the number ofpending peer-to-peer requests is less than the first number, allowingthe number of clients to issue a peer-to-peer request.
 19. Theintegrated circuit of claim 18 wherein the first number is one.